1. Field of the Invention
The present invention relates to continuous wave frequency-modulated (FM-CW) ranging systems and, in particular, to a modulation rate and frequency excursion drift compensator for use in such systems.
2. Description of the Prior Art
FM-CW radar ranging interrogating systems are well known in the art. In such systems, a radio frequency (RF) interrogation signal, frequency modulated with a given modulation waveform, is transmitted toward a target and reflected therefrom back to the interrogating system. The reflected signal as received at the interrogating system is delayed in time, and hence shifted in frequency, from the instantaneous interrogation signal by an amount (.tau.) proportional to the range (R) of the target. For example, when the interrogation signal is modulated by a triangular waveform having a peak value of .increment.F, a period of 1/f.sub.m, the frequency shift or difference frequency f.sub.R (also commonly known as beat frequency) as generated by a suitable filtered mixer receptive of the reflected signal and a signal indicative of the interrogation signal, is equal to the time derivative of the frequency of the interrogation signal times the round trip time delay, .tau., and may be expressed as: EQU f.sub.R = df/dt .multidot. .tau. = 4R (.increment.F .multidot. f.sub.m)/C (1)
where C is the speed of light. The range, R, or distance between the target and the interrogating system and, additionally, the range rate of the target relative to the system is determined by measurement of the frequency shift f.sub.R. Rearranging formula (1) provides the formula for range. EQU R = f.sub.R .multidot.C/4 (.increment.F .multidot.f.sub.m) (2)
or EQU R = K .multidot. f.sub.R ( 3)
where EQU K = C/4 (.increment.F .multidot. f.sub.m)
if .increment.F and f.sub.m remain constant, K remains constant.
In practice, such FM-CW radar ranging systems have been plagued with errors due to drifts in the maximum frequency excursion (.increment.F) and/or period (1/f.sub.m) of the frequency modulation waveform. Such drifts are often caused merely by changes in ambient conditions such as temperature, power supply variations and timing circuit drifts. U.S. Pat. Nos. 3,968,492 issued July 6, 1976, to G. Kaplan and 3,974,501 issued Aug. 10, 1976, to A. Ritzie, disclose signal processors for FM-CW ranging systems which make provisions for compensating for drifts in the period (1/f.sub.m) of the modulation waveform. However, no provision is made for compensating for drifts in the frequency excursion (.increment.F) of the modulation waveform. The prior art has typically used one of two methods of compensating for drifts in frequency excursion. In one such prior art method, while the system is off line, a delay line of known length is temporarily connected between the transmitter and receiver antenna feedlines. The time delayed signal, generated by the delay line from a portion of the interrogation signal simulates a reflected signal from a target at a predetermined range. The processor is then manually adjusted to display the predetermined range and the delay line thereafter disconnected. Such a method is limited in use in that any subsequent changes in the modulation frequency or amplitude which may be undetected require recalibration of the system, again while it is off line.
A second prior art method utilizes a delay line in conjunction with a frequency discriminator in a complex closed loop stabilizing system. Such a system is described in "The Radar Handbook", M. I. Skolnik, McGraw Hill, 1970, chapter 16, pages 29-32. However, frequency discriminators are relatively expensive and not suitable for large volume usage. Such frequency discriminator systems maintain a constant frequency excursion .increment.F, but do not provide an accurate calibration for the drifts of f.sub.m, the modulation waveform frequency. Such a ranging system must additionally employ, for example, relatively expensive crystal oscillators, temperature controlled ovens, or both U.S. Pat. No. 4,008,475 issued Feb. 17, 1977, to the present inventor uses a delay line in a feedback network to alter one of .increment.F or f.sub.m to keep their product constant.